Data storage devices having moving recording media, such as rotating magnetic discs, access data stored on the media through a transducer known as a read and/or write head. The transducer is positioned in close proximity to the moving media. For performance and reliability reasons, it is important to maintain a controlled spacing between the transducer and the recording media. Head-Media Spacing (HMS) modulation is a measure of the displacement of the transducer, or head, relative to the media.
Some data storage systems employ active clearance control, wherein one or more elements of the transducer or the transducer's support structure are movable relative to the medium surface. For example, microactuators have been positioned along the transducer's suspension for displacing the transducer vertically relative to the medium. Also, the characteristics of thermal pole tip protrusion have been used for active clearance control. Differences in thermal coefficients of expansion between the materials used for forming the write pole and other elements of the transducing head can cause movement of the write pole relative to the other elements with changes in temperature. By controlling the temperature of the write pole, protrusion of the write pole relative to the other elements can be controlled to thereby control the spacing between the write pole and the medium surface. For example, thermal pole tip protrusion can be increased by elevating the temperature of the write pole material by some method, such as by turning on or elevating a write current.
Active clearance control methods adjust the temperature of the write pole based on some measure of the Head-Media Spacing. A variety of methods have been used for detecting or measuring the Head-Media Spacing. For example, U.S. Pat. No. 7,038,875 describes a dynamic measurement of Head-Media Spacing modulation, which is based on a harmonic ratio measurement of the recording head read-back signal. Typical harmonic ratio measurements need to be calibrated at zero spacing, which requires the write pole to contact the medium surface. As the write pole is actuated toward the medium surface, contact has been detected by several methods.
One current technique for rotating-disc type medium is to detect off-track movement of the head caused by friction at the contact when the head is flying at a non-zero skew angle. However, such friction can cause undesirable wear on the head and/or recording surface. For a perpendicular recording head, the write pole dimension can be less than 200 nanometers. It has been found that the tip of the write pole is often severally worn before the contact can be reliably detected. Therefore a dedicated feature, such as a contact pad, has been fabricated on the head to generate enough interaction at the contact to allow detection of the contact while preventing the write pole tip from being worn out. However, the contact pad is typically manufactured using different materials (e.g., non-magnetic) than the write pole and is typically positioned at a certain distance away from the write pole. Therefore, significant height variations can exist between the write pole and the contact pad after typical lapping and milling processes. These variations are becoming a major limitation for active clearance control since these variations often cannot be compensated by the write pole heater. A further limitation of the current detection technique is that the technique cannot detect the contact when the head is oriented at a zero skew angle relative to the tangential velocity of the disc since there is no off-track movement of the head caused by the friction at the contact.
Accordingly, alternative contact detection techniques are desired that may be capable of reliably detecting contact at the write pole without significant wear and that do not require a contact pad and its resulting head medium spacing loss caused by height variations between the write pole and the contact pad.
Embodiments of the present disclosure provide solutions to these and other problems, and offer other advantages over the prior art.